In recent years, supercritical fluid chromatography (hereinafter “SFC”) has been gaining attention. The SFC is chromatography that is performed by applying specific temperature and pressure to carbon dioxide or the like to obtain a supercritical fluid, and by using the supercritical fluid as a solvent. The supercritical fluid has properties of both liquid and gas, and is more diffusive but less viscous than liquid. Using such a supercritical fluid as a solvent allows fast, high-separation, high-sensitivity analysis,
Generally, to maintain a solvent in a supercritical state, the flow rate has to be a minute flow rate of 3 ml/min or less, and the pressure in a flow system has to be 10 MPa or more. To this end, an SFC device has a pressure control valve for maintaining the flow system at a specific pressure of 10 MPa or more provided at a later stage of an analytical column.
The pressure control valves include those that adopt a method of adjusting a gap (opening area) between a valve seat provided with an inlet channel and a valve body blocking the inlet channel (see Patent Document 1), those that adopt a method of adjusting a channel width (opening area) by driving a diaphragm (see Patent Document 2), those that adopt a method of inserting a needle into an orifice opening at an end of an inlet channel and of adjusting a gap (opening area) based on the insertion depth of the needle into the orifice opening (see Patent Document 3), and the like.
Furthermore, a diaphragm valve may be cited as a general control valve which is not for SFC (see Patent Documents 4 and 5). Generally, a diaphragm valve has an inlet channel arranged at the center of a valve chamber, and controls the pressure by pressing a valve body (diaphragm) against an orifice valve seat of the inlet channel and blocking the inlet channel, and also an outlet is arranged at one position in the periphery. According to this structure, the internal volume of a control chamber is great, and a volume (dead volume) that does not contribute to the flow of a liquid is present on the opposite side of an outlet channel from the inlet channel.
Although it is not a valve for SFC, a pinch valve may be cited as a valve with a small dead volume (see Patent Document 6). The pinch valve controls the pressure by having an inlet tube and an outlet tube inserted to both ends of a deformable valve tube, and by adjusting the opening areas communicating with the inlet tube and the outlet tube by flattening the valve tube. To control the pressure inside a channel of the SFC, a high sealing property is required at insertion portions of an inlet tube and an outlet tube to a valve tube. Since a pressure of 10 MPa or more is sometimes applied inside the channel, and also the inner diameter of the channel is generally 0.3 mm or less so that a mobile phase is made to flow at a minute flow rate of 3 ml/min or less, it is difficult to connect a pipe whose inner diameter is 0.3 mm or less to the valve tube with a high sealing property.
Moreover, as described above, with the SFC, the pressure generally has to be at 10 MP or more in a state where the flow rate of a mobile phase is 3 ml, min or less, and thus, opening/closing control is to be performed in a state where the valve tube is almost completely flattened and the opening area is 0.001 mm2 or less. This may cause a stress to concentrate at the flattened portion of the valve tube, and the valve tube may be damaged. Accordingly, it is difficult to use such a pinch valve for the SEC.